The present invention relates to an electromagnetically driving part for use in a vehicle braking system for performing antilock control, traction control, and so on.
First, the schematic configuration of a vehicle braking system using an electromagnetically driving part according to the present invention will be described with reference to FIG. 9. A left front wheel brake Bfl, a right rear wheel Brr, a right front wheel brake Bfr and a left rear wheel brake Brl are disposed in the respective wheels. In addition, a pair of master cylinders M/C form a tandem master cylinder so as to output braking fluid pressure corresponding to the depression of a brake pedal. A braking fluid pressure controller 100 is provided among the brakes Bfl, Brr, Bfr and Brl and the tandem master cylinder M/C so as to be connected with the brakes Bfl, Brr, Bfr and Brl through a main fluid-pressure channel II and with the master cylinder M/C through a main fluid pressure channel I.
A braking fluid pressure controller 100 has four normally open electromagnetic valves Vofl, Vorr, Vofr and Vorl; four check valves Cofl, Corr, Cofr and Corl; four normally closed electromagnetic valves Vcfl, Vcrr, Vcfr and Vcrl; a pair of reservoirs Ra and Rb; a pair of reciprocating plunger pumps Pa and Pb; and a plurality of fluid pressure channels for making these members communicate with one another. The normally open electromagnetic valves Vofl, Vorr, Vofr and Vorl are provided as valve means corresponding to the left front wheel brake Bfl, the right rear wheel Brr, the right front wheel brake Bfr and the left rear wheel brake Brl. The four check valves Cofl to Corl are connected in parallel with the normally open electromagnetic valves Vofl to Vorl respectively so as to be opened to send brake fluid from the corresponding wheel brakes Bfl to Brl back to the master cylinder M/C when the input to the master cylinder M/C is released. The normally closed electromagnetic valves Vcfl to Vcrl are provided individually correspondingly to the respective wheel brakes Bfl to Brl. The reservoirs Ra and Rb are provided respectively correspondingly to the left front wheel brake Bfl and the right rear wheel Brr and to the right front wheel brake Bfr and the left rear wheel brake Brl. The reciprocating plunger pumps Pa and Pb are connected with the reservoirs Ra and Rb respectively.
In addition, the braking fluid pressure controller 100 has a single braking fluid pressure controller motor (hereinafter referred to as a xe2x80x9cmotorxe2x80x9d simply) M for driving both the plunger pumps Pa and Pb, and an electronic control unit ECU for controlling the switching of the respective normally open electromagnetic valves Vofl to Vorl and the respective normally closed electromagnetic valves Vcfl to Vcrl between demagnetization and excitation.
Running conditions of a vehicle are fed into the electronic control unit ECU from vehicle velocity sensors (not shown) provided in the respective wheels, and so on. In accordance with the running conditions of the vehicle, the electronic control unit ECU controls each of the wheel brakes Bfl to Brl on the basis of any one of the following three commands In a pressure increasing mode, one of the normally open electromagnetic valves Vofl to Vorl corresponding to the wheel in question is demagnetized and opened, and one of the normally closed electromagnetic valves Vcfl to Vcrl corresponding to the wheel in question is demagnetized and closed. Thus, the pressure is increased by the fluid pressure applied from the master cylinder M/C to a wheel cylinder (not shown). In a retaining mode, one of the normally open electromagnetic valves Vofl to Vorl corresponding to the wheel in question is excited and closed, and one of the normally closed electromagnetic valves Vcfl to Vcrl corresponding to the wheel in question is demagnetized and closed. Thus, the fluid pressure is prevented from being transmitted from the master cylinder M/C to a wheel cylinder (not shown), so that the fluid pressure is retained. In a pressure reducing mode, one of the normally open electromagnetic valves Vofl to Vorl corresponding to the wheel in question is excited and closed, and one of the normally closed electromagnetic valves Vcfl to Vcrl corresponding to the wheel in question is excited and opened. Thus, the braking fluid is reserved temporarily from a wheel cylinder (not shown) into the reservoirs Ra and Rb, so that the fluid pressure is reduced to prevent the wheel from being in a lock state.
Then, the electronic control unit ECU drives the motor M so as to drive the pumps Pa and Pb. Thus, the braking fluid reserved temporarily in the reservoirs Ra and Rb is sent back to the master cylinder M/C.
Next, a background-art configuration of such a normally open electromagnetic valve used in a vehicle braking system will be described with reference to FIG. 10. In FIG. 10, the reference numeral 10 represents a housing member or main body formed out of magnetic metal and shaped into a stepped cylinder. The housing member 10 is fitted into an electromagnetic valve fitting hole 12 formed in a substrate 11, and attached to an inner surface of the fitting hole 12 while being sealed fluid-tightly by using a seal member 13. The reference numeral 14 represents a snap ring for preventing the housing main body 10 from falling off. On one side of the housing main body 10, a cylindrical fixed core 15 of an electromagnetic coil 30 is provided to extend toward the outside of the fitting hole 12. The reference numeral 16 represents a communication path for making the aforementioned main fluid pressure channels I and II communicate with each other.
The reference numeral 17 represents a guide cylinder fixed to the fixed core 15. The guide cylinder 17 is formed out of non-magnetic material, for example, stainless steel, into a thin closed-end cylinder the front end of which is formed as a spherical closed end. The guide cylinder 17 is engaged with the external surface of the fixed core 15 by welding. The reference numeral 18 represents a movable core which is slidably received in the guide cylinder 17. A valve shaft 19 of non-magnetic material is inserted into the fixed core 15 slidably, and disposed so that a valve ball 20 provided as a valve body on the front end of the valve shaft 19 faces a valve hole 23 of a valve seat member 22. The valve shaft 19 is always in a position where the valve ball 20 is detached from the valve seat member 22 by the energizing operation of an elastic body 24, that is, in a position where the valve is opened. By the operation of magnetic flux generated when a current is made to flow through the electromagnetic coil 30, the movable core 18 is sucked and driven so that the valve ball 20 abuts against the valve seat member 22 and reaches the position where the valve is closed.
FIG. 11 shows in magnification the portion where the guide cylinder 17 is engaged with the fixed core 15. A press-in portion 15a is provided on the front end of the fixed core 15 so that the guide cylinder 17 is press-fitted onto the press-in portion 15a. After the guide cylinder 17 is press-fitted onto the press-in portion 15a, the guide cylinder 17 is welded with the press-in portion 15a from the external surface of the guide cylinder 17 and over the entire surface thereof so that the fixed core 15 and the guide cylinder 17 are seal-engaged with each other. A welded portion 40 shows a portion which was welded at the time of welding.
The process where the aforementioned welded portion 40 is formed will be described. The portion 40 is melted by heat at the time of welding, and metal vapor (gas) is generated then. There is a possibility that the metal vapor catches the molten metal and flies about when the metal vapor leaves the portion 40. Thus, there appears a phenomenon that the flying metal (sputtering) adheres to the external surface of the guide cylinder 17 or the portion 40 is formed to swell over the external surface of the guide cylinder 17 due to the surface tension of the molten metal when the molten metal is solidified. In the background art, taking such a phenomenon into consideration, a certain measure of clearance is provided between the internal surface of the electromagnetic coil 30 and the external surface of the guide cylinder 17. However, if this clearance is made large, the magnetic efficiency of a magnetic path in the electromagnetically driving part is lowered so that the electromagnetically driving part is prevented from being miniaturized in its radial direction.
In addition, a distance A (lift quantity) between the valve ball 20 and the valve seat member 22 in the electromagnetically driving part and a distance B (air gap) between the end surface of the fixed core 15 and the end surface of the movable core 18 are very important. It is necessary to assemble the electromagnetically driving part with high precision such that the allowable dimensional tolerances of the respective distances A and B are within 0.1 mm. In addition, the distances A and B are quantities related with each other. The sizes of the distances A and B are set in the relation xe2x80x9cA less than Bxe2x80x9d. The reason why the distances A and B are set thus will be described. If the relation xe2x80x9cA greater than Bxe2x80x9d is established, the movable core 18 is driven and sucked to the fixed core 15 when a current is made to flow through the electromagnetic coil 30. Although the valve shaft 19 ought to be pushed so that a contact seal is established between the valve ball 20 and the valve seat member 22, the fixed core 15 and the movable core 18 abut against each other earlier so that suction force is not transmitted to the valve shaft 19. Thus, such a contact seal cannot be established, and the OFF responsibility deteriorates when the electromagnetically driving part is demagnetized.
The suction force acting on the fixed core 15 changes in a quadratic curve due to the aforementioned distance B (air gap). Therefore, the size of the distance B affects the ON-OFF responsibility on a large scale to thereby cause a failure in performance. Shrinkage in welding also causes a variation in the distances A and B to affect the tolerances of the distances A and B. The shrinkage in welding means that the guide cylinder 17 shrinks to compensate a volume caused by the above-mentioned vaporization and flying-about of metal at the time of welding. It is therefore necessary to take the quantity of shrinkage after welding into consideration beforehand when the respective guide cylinder 17 and valve seat member 22 are pressed onto and into the fixed core 15 so as to be assembled therewith. Incidentally, this quantity of shrinkage also varies to some extent.
On the other hand, there are various factors of reduction in the press-in accuracy of the electromagnetically driving part as follows. Deformation is caused when the guide cylinder 17 is press-fitted; the guide cylinder 17 may be press-fitted slantingly with respect to its axis; when the guide cylinder 17 is press-fitted, it comes back from its press-fitted position due to spring-back; a scattering may be caused in a press-fitting load due to a scattering in the appearance of the external surface of the fixed core 15 or a sliding resistance thereof; a scattering may be caused in measuring the dimensions of parts even if the press-fitted position of the guide cylinder 17 is adjusted in accordance with the dimensions of the parts measured beforehand when the guide cylinder 17 is press-fitted; and so on.
Of these factors, in order to increase the press-in accuracy of the guide cylinder 17, it is effective to reduce the press-fitting load when the guide cylinder 17 is press-fitted. Therefore, the front end portion of the press-in portion 15a is reduced in diameter so that a guide portion 15b for guiding the guide cylinder 17 is provided to take measures to prevent the press-fitting load from increasing. In addition, it is preferable to make the guide portion 15b longer, in such a manner that the guide portion 15b makes it easier to guide the guide cylinder 17 and prevents the guide cylinder 17 from inclining.
As another measurement for reducing the press-fitting load, such a method may be considered that the length of the press-in portion 15a is made as short as possible. However, when the guide cylinder 17 is welded with the outer circumference of the press-in portion 15a in consideration of scattering in dimensions of welding equipment, support jigs, and constituent parts relating to the welding position, the press-in portion 15a has to have a certain length.
As has been described, in the background-art electromagnetically driving part, the guide portion 15b and the press-in portion 15a have to have a certain length which is long enough to press the guide cylinder 17 into the fixed core 15 with precision and weld the guide cylinder 17 with the press-in portion 15a. Thus, the fixed core 15 is prevented from being miniaturized. In addition, in terms of the diameter direction of the electromagnetically driving part, it is necessary to allow a clearance between the external surface of the guide cylinder 17 and the internal surface of the electromagnetic coil 30. Thus, the electromagnetically driving part is prevented from being miniaturized.
The present invention was achieved to solve these problems. It is an object of the present invention to provide an electromagnetically driving part in which the dimensions in the axial direction and in the radial direction are shortened so that the electromagnetically driving part can be miniaturized while improving the accuracy in assembling, and in which a guide cylinder and a fixed core are surely engaged with each other by welding so that the reliability is high.
To attain the foregoing object, according to an aspect of the present invention, there is provided an electromagnetically driving part for use in opening/closing a valve or the like, comprising a guide cylinder in which a movable core is received slidably, and a fixed core fixed to the guide cylinder, the fixed core and the guide cylinder being engaged with each other by welding in a state that the guide cylinder is press-fitted onto the fixed core; wherein an air gap is formed between an outer circumferential surface of the fixed core and an inner circumferential surface of the guide cylinder; and wherein the fixed core and the guide cylinder are welded with each other in a portion where the air gap is formed. Because the fixed core and an internal surface of the guide cylinder are welded with each other in the portion where an air gap is formed, the welded portion is not formed to swell over the external surface of the guide cylinder. Accordingly, the clearance between the guide cylinder and the internal surface of an electromagnetic coil inserted onto the guide cylinder can be set to be small. In addition, sputtering is prevented at the time of welding, so that the bonding can be performed with high reliability.
Preferably, in the above electromagnetically driving part, a front end of a press-in portion of the fixed core onto which the guide cylinder is press-fitted is reduced in diameter so that an air gap is formed between the outer circumferential surface of the fixed core and the inner circumferential surface of the guide cylinder; and the fixed core and the guide cylinder are welded with each other in a portion where the air gap is formed. Accordingly, the axial length of the fixed core is reduced so that the electromagnetically driving part can be miniaturized.
Preferably, in the above electromagnetically driving part, the inner circumferential surface of the guide cylinder at its front end is enlarged in diameter so that an air gap is formed between the outer circumferential surface of the fixed core and the inner circumferential surface of the guide cylinder; and the fixed core and the guide cylinder are welded with each other in a portion where the air gap is formed. Accordingly, the shrinkage of the guide cylinder generated at the time of welding can be absorbed on the open end side of the guide cylinder. Therefore, the accuracy in attachment of the guide cylinder and a valve seat member can be improved.
Preferably, in the above electromagnetically driving part, the guide cylinder and the fixed core are welded with each other by laser welding. Accordingly, the guide cylinder and the fixed core are engaged with each other easily and surely.